We did have a little discussion over this one, about how could it work.

I figure it would work like a mix between the shotgun shells on springers that hold multiple bb's but don't shoot them all at the same time, and gas grenade shells.
the biggest problem would be short barrel and the big striker pin/bolt that moves when you're firing, so you'd probably have to make the shell a bit longer than it really is to hold more gas for the heavy mechanism (and make the mechanism as light as possible).
The coolest part is that it reloads itself, since those shells are held inside that magazine and I think that the magazine itself is also detatchable. (mag inside a mag yo, just like the actual AR2)
The auto reload could be either fully automatic or triggered by the user pressing a key or removing the shell (you can see two extractors on the left)

the secondary fire would function so that one of the shells inside the magazine gets fully unloaded in a very fast burst and then ejected out of a side port (or left inside, but then the rifle would start loading empty shells)
or simply "placed" on a side area so that it isn't in the way/doesn't fall out/can be retrieved by hand

That's almost impossible to work because there's nowhere to put the gearbox. You could maybe fit a v6 in the part that you lift up to put the mag in. I don't know how you would adjust the hop up because it would be really far back from the bolt. And it would probably need a custom battery like the Real Sword Type 97.

Actually guys, it could work
Red is the barrel, Orange is the bolt, the green is the entirety of the bolt travel (it's whole length), blue is the recoil spring, purple is where the bb is seated in the magazine and pink is where the recoil gas would go (it would go towards the front then go up and push the bolt back and the bb forward). The BB would simply travel very far before it got to the barrel, meaning that the ROF would be shite but there you go.

in fact the magazine would probably have a realcap amount (or maybe double stack) and the rest would be for the gas

Edited:

an alternative would be a reverse mechanism, where the bolt is actually pushed forward

I'm still trying to figure out what could have compelled a Frankenstein of guns to stick a pump shotgun grip on a FAMAS, ram a drum into the stock, and call it a day. Isn't there a Killzone gun that's a bastardization of an M14 and SPAS-12?

I had an idea for a laser rifle before. It would use MF cell look alikes which would be hicap mags, it would flow down the back and then forward a little bit, where it would then be shot out where the hole for the laser would be on the laser rifle itself.The battery would be housed in the handgrip like part before the grip-area under the wire frame slants into the main body.